Magnets
<p>
Fill each domino shape with either a magnet (consisting of a + and
&#8722; pole) or a neutral domino (green).
<p>
The number of + poles that in each row and column must match the
numbers along the top and left; the number of &#8722; poles must
match the numbers along the bottom and right. Two + poles may not be
orthogonally adjacent to each other, and similarly two &#8722; poles.
<p>
Left-click a domino to toggle it between being empty and being a
magnet (the + is placed in the end you click). Right-click to toggle
between empty, neutral, and a ?? mark indicating that you're sure
it's a magnet but don't yet know which way round it goes.
<p>
Left-click a clue to mark it as done (grey it out). To unmark a clue
as done, left-click it again.
